The mechanisms whereby the pigment epithelium (PE) recognizes, engulfs, and digests the worn out membranous disks of the photoreceptors will be studied in bovine and human PE cells in vitro. Objects for phagocytosis will include: outer segment disks; inert materials; radioactively-labeled particles; and particles with enzyme-modified surface properties. Optimal conditions for phagocytosis will be established and the system will be used to study the cellular biology of phagocytosis in PE cells. Changes in the pH of the phagosome and in metabolic pathways of the PE will be determined. The effects of enzymatic removal of cell coat components will be assessed, and binding sites will be studied using lignins. Quantitative aspects of uptake of exogenous radioactive particles and the release of soluble catalytic products will be determined using scintillation techniques. Retention of insoluble products in the cell will be visualized by electron microscopic (EM) autoradiography. The role of microtubules and microfilaments in phagocytosis will be studied. Fractionation of PE cells will be carried out using differential and sucrose density gradient centrifugation. Fractions will be studied for enzyme localization by EM histochemistry. The ultrastructure of human subretinal fluid will be studied and the synthetic capabilities of the various cellular components will be determined by EM autoradiography. The phagolysomal system of these cells will be studied by EM histochemistry.